1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly to improvement of image display in a liquid crystal display having a storage capacitor (referred to as "SC" hereinafter).
2. Description of the Related Art
Vertical orientation type liquid crystal displays using liquid crystal with negative dielectric constant anisotropy and vertical orientation films have been developed. Devices of this type can be classified into two groups.
Devices in the first type use a vertical orientation film that has been treated by rubbing processing. FIG. 1A is a plan view showing an example of this type, and FIG. 1B shows a cross-sectional view taken along line 1B--1B of FIG. 1A. Gate lines 51 are formed on a first substrate 50, and a gate insulating film is formed covering the gate lines 51. Each gate line 51 includes a gate electrode 52 within a portion of a pixel. Storage capacitor electrodes (SC electrodes) 53 composed of amorphous silicon (a-Si) film are formed in discrete islands in an overlying layer so as to cross over the gate electrode 52. The SC electrode 53 is doped with impurities, and, together with the gate electrode 52, forms a thin-film transistor (TFT). These layers are covered by an interlayer insulating film 54. A pixel electrode 55 composed of ITO (indium tin oxide) is formed on the interlayer insulating film 54, and is connected to the SC electrode 53 via a contact hole opened in the interlayer insulating film 54. Although such a contact actually is not present in this cross-section, contact is shown in the cross-sectional view illustrated in FIG. 1B shows this contact to help understanding. In the next overlying layer, a vertical orientation film 56 is formed. The vertical orientation film 56 has been treated by rubbing processing. The interlayer insulating film 54 is composed of two layers, and a data line 57 is disposed in the middle of the interlayer insulating film 54. The data line 57 is connected to a source region of the TFT and supplies electric charge to the SC electrode 53 and the pixel electrode 55 when the TFT is turned on. The data line 57 is formed in a position underneath the pixel electrode 55 so as to form a vertical overlap.
On a second substrate 60 opposing the first substrate 50, a common electrode 61 composed of ITO and other materials is formed covering the plurality of pixel electrodes 55. Over the common electrode 61, a vertical orientation film 62 identical to the one disposed on the first substrate 50 is deposited and treated by rubbing processing.
Liquid crystal 70 is sealed between the first substrate 50 and the second substrate 60. The orientation of the liquid crystal molecules are controlled according to the strength of the electric field generated by a voltage applied between the pixel electrode 55 and the common electrode 61. On the outboards of the first substrate 50 and the second substrate 60, polarizing plates (not shown) are arranged such that their polarization axes are perpendicular to one another. The linearly polarized light passing between the polarizing plates is modulated while passing through the liquid crystal 70 controlled in different orientations in the respective display pixels, and is thereby controlled to a desired transmittance.
The liquid crystal 70 has negative dielectric constant anisotropy, i.e., its molecules tend to tilt towards the direction of the electric field. The vertical orientation films 56, 62 control the initial orientation of the liquid crystal in the vertical direction. Accordingly, when no voltage is applied, the liquid crystal molecules are oriented vertically with respect to the plane of the vertical orientation films 56, 62, and linearly polarized light passing one of the polarizing plates passes through the liquid crystal layer 70 but is then obstructed by the other polarizing plate, resulting in a black display. When a voltage is applied, the molecules of the liquid crystal 70 align in the rubbing axis. Consequently, the linearly polarized light that passed one of the polarizing plates is subjected to birefringence in the liquid crystal layer 70, is changed into an elliptically polarized light, and passes through the other polarizing plate. The display then approaches white. When both the gate line 51 and the data line 57 are turned on, a voltage is applied to the pixel electrode 55 via the TFT, and the liquid crystal positioned directly above the pixel electrode 55 is driven. An image is generated on the LCD by the application an independent voltage to each of the pixel electrodes 55. In other words, a region in which a pixel electrode 55 is formed is defined as a pixel.
A light-blocking black matrix (not shown) is formed in regions other than pixels, i.e., gaps between the pixel electrodes 55 and the regions constituting the TFT including the SC electrodes 53. The black matrix is disposed to prevent white inter-pixel regions from reducing contrast. When light transmitted through one of the polarizing plates and coming into the liquid crystal layer is subjected to birefringence when passing through the pre-tilted crystal, the black matrix prevents undesired light from irradiating through the other polarizing plate in the inter-pixel regions where no voltage is applied.
The function of the SC electrode 53 is next explained. In the LCD, a voltage is applied between the pixel electrode 55 and the common electrode 61, and transmittance is controlled by orienting the liquid crystal using the electric field generated by the applied voltage, as described above. However, as liquid crystal is not an absolute insulator, a slight current flows when the voltage is applied to the pixel electrode. Consequently, the electric charge stored in the pixel electrode 55 becomes discharged, and the voltage between the pixel electrode 55 and the common electrode 61 cannot be maintained. To solve this problem, a storage capacitor (SC) line 58 made of chromium or similar material is disposed to form a storage capacitor together with the SC electrodes 53 in the portions overlapping the SC electrodes 53, thereby supplying electric charges to the pixel electrode 55. The SC line 58 is formed to have a large width at the portion 58a opposing the SC electrode 53 to provide a large capacitance together with the SC electrode 53. FIG. 2 shows an equivalent circuit including a pixel, a storage capacitor, a data line, and a gate line. The capacitor constituted by the liquid crystal 70 interposed between the pixel electrode 55 and the common electrode 61, and the storage capacitor constituted by the SC electrode 53 and the SC line 58, are connected to the data line 57 via the TFT including the gate electrode 52.
In the second type of vertical orientation type LCD, the vertical orientation film is not treated by rubbing processing. Instead, the vertical orientation type LCD of the second type comprises a separate orientation control means for controlling the liquid crystal orientation. A vertical orientation type LCD having orientation control windows for controlling orientation is proposed in the commonly assigned Japanese Patent Application No. H05-84696 (JPA H06-301036), for example. FIG. 3A is a plan view illustrating an LCD having orientation control windows, and FIG. 3B is a cross-sectional view taken along line 3B--3B of FIG. 3A. The LCD of FIGS. 3A and 3B coincides with the LCD of FIGS. 1A and 1B in that an SC electrode 53 forming a TFT and a pixel electrode 55 connected to the SC electrode are provided on a first substrate 50 which together with a substrate 60 seals liquid crystal 70, and that polarizing layers are disposed on the outboard. Structures of FIGS. 3A and 3B that correspond to structures of the LCD of FIGS. 1A and 1B are labeled with corresponding reference numerals and their explanations will not be repeated. The LCD of FIGS. 3A and 3B greatly differs from the LCD of FIGS. 1A and 1B in that openings are made in the common electrode 61 to form orientation control windows 63, and that the vertical orientation films 56, 62 are not treated with rubbing processing. An orientation control window 63 is a region where no electrode is present, and is shown in the example in the shape of two "Y's" connected at their bottoms.
In this arrangement, a voltage is applied between the pixel electrode 55 and the common electrode 61 to create electric fields 64, 65 which tilt the liquid crystal molecules 70. At an end portion of the pixel electrode 55, the electric field 64 slants from the pixel electrode 55 towards the common electrode 61. Similarly due to the absence of electrode, the electric field 65 slants towards the pixel electrode 55 at an edge of an orientation control window 63. The orientation of the liquid crystal is controlled by these slanted electric fields. The liquid crystal molecules therefore towards the orientation control window 63 regardless of their pre-tilt angle.
Due to the absence of common electrode 61 in a region directly beneath the orientation control window 63, no electric field is created even during the voltage application between the electrodes. The liquid crystal molecules in this region are therefore fixed in the initially oriented state, i.e., the vertical direction. Accordingly, regions of the liquid crystal on the respective sides of the orientation control window 63 can be oriented in opposing directions via the continuous property of liquid crystal, resulting in a broader viewing angle than in the LCD of FIGS. 1A and 1B.
(Additional Art)
A black matrix is not absolutely necessary in a vertical orientation type LCD without rubbing processing because such LCD adopts the "normally black scheme" wherein black is displayed when no voltage is applied. Eliminating a black matrix is disclosed in, for example, Japanese Patent Application No. H09-317169, which was filed by the present applicant and does not constitute prior art for the present invention.
FIGS. 4A and 4B illustrate another example of an LCD of the second type. In this example, the data line 59 is formed overlapping the orientation control window 63. Light transmitting through the data line 59 attenuates by a fixed ratio. In addition, the liquid crystal beneath the orientation control window 63 does not allow light to pass through even when voltage is applied because the initial orientation is maintained. Light transmittance is reduced in these regions, resulting in a large decrease of light transmittance in the overall pixel. To alleviate this problem and minimize any decrease in light transmittance, these regions are formed so as to overlap. Details concerning this point are described in Japanese Patent Application No. H10-337840, which was filed by the present applicant and does not constitute a prior art for the present invention.
As described above, a storage capacitor (SC) line is provided under the SC electrode 53. The SC line is applied with a voltage so as to supply electric charges to the pixel electrode 55.
However, in such an arrangement, the voltage applied to the SC line leaks between adjacent pixels into the liquid crystal layer, thereby allowing light to pass through the liquid crystal located between the pixels. In the above-mentioned vertical orientation type LCD without black matrix, no structure is provided to block the inter-pixel transmitting light. Deterioration in display quality, such as decreased contrast, may therefore result, especially when displaying the color black.
Further, as the liquid crystal molecules driven in inter-pixel locations are not under orientation control, their orientation directions are diverse, differing in individual cells and after each application of voltage. In an LCD wherein rubbing processing is not performed on the vertical orientation films 56,62, the control over liquid crystal orientation is relatively weak. The orientation of the liquid crystal located within a pixel and driven by a voltage in the pixel electrode is therefore disturbed by the inter-pixel liquid crystal due to the continuous property of liquid crystal, resulting in deterioration of display quality.